This invention relates to swept group delay measurements, and more particularly to a method and apparatus for frequency sweeping a system, which may be a microwave component, and recording group delay directly as a function of frequency.
Group delay measurements are of increasing importance for high resolution ranging systems at S- and X-band. Group delay is defined as the time required for an information signal to pass through a system and can be measured as the phase shift, .DELTA..phi., of the information signal through the system for a given input frequency, .omega.. This suggests shifting the input frequency a predetermined amount, .DELTA..omega., recording the resulting change of phase, and repeating the process over the desired range of frequencies. Alternatively, the input signal may be amplitude modulated by a lower modulating frequency, fm. After the carrier passes through the system, the amplitude modulation is detected and compared with the phase of the modulating frequency to determine the group delay, td, of the carrier as simply ##EQU1## By using a modulation frequency of 2.7778 MHz, which is small compared to the test band widths, a direct conversion from degrees of phase to nano-seconds can be made as illustrated in the following example of a phase measurement .phi. of 1.degree. with modulation of 2.7778 MHz; ##EQU2## The problem with this direct approach is that phase shifts or group delay effects of the modulator and detector in the test path are included in the measurement. Such effects can be predetermined and subtracted from the measured group delay, but changes in these effects as a result of temperature changes, or as the result of carrier frequency changes, may introduce error since such effects are not present in the reference signal path.